The invention relates to a method and a device for testing consistent test objects for the presence of holes or cracks in a bottom surface, wherein each test object is bounded off by the bottom surface, an opening situated opposite the bottom surface, and a lateral surface extending between the opening and the bottom surface.
Typical test objects are for example plastic closure caps which are used to seal containers so as to be air-tight. Such closure caps may have a guarantee strip in the region of the opening edge, the so-called freshness seal, which is detached from the closure cap when opened. In order to facilitate this detaching, the guarantee strip is connected to the closure cap only by thin webs. The guarantee strip can be produced directly during the injection molding process of the closure cap. Another possibility is to make slits in the closure cap which are evenly distributed about the circumference, at slight distance from the opening edge. Moreover, closure caps are known with several flaps secured by webs to their opening edge, which can be brought to bear against the inside of the closure cap when screwed on. Such closure caps owing to the manufacturing process have openings between the guarantee strip or the'flaps and the lateral surface.
Plastic closure caps are usually produced as injection molded parts. During manufacture, leaky areas known as microholes or microcracks may occur in the bottom surface near the injection point. However holes and cracks must be avoided in the closure cap of a container, in order to prevent an unwanted escaping of the contents from the container closed with the closure cap or an unwanted entry of gases or liquids into the container.
Hence there is a need to automatically identify and sort out test objects, especially plastic closure caps, with such defects.
JP S64-046 622 A discloses a method for testing containers for tightness, in which the containers are transported continuously by means of a conveying mechanism in a transport plane, and by means of a high-voltage source a test voltage is applied between a stationary concentration electrode situated above the transport plane and a stationary side electrode situated in the transport plane when the container to be tested is found between the electrodes. A leak in the container is identified if a breakdown between the electrodes on account of the changing fill level in the container is identified by a current measuring device.
DE 32 13 100 A1 discloses a method of finding and sorting out defective ceramic objects in which the flaws are determined by electrostatic charging. The ceramic objects are moved continuously through a test station where they are subjected to consecutive high-voltage discharges, so that basically all areas of the ceramic objects are covered by the high-voltage discharges. The electrodes are arranged underneath the trajectory of the ceramic objects to be tested and correspond in their shape to the shape of the ceramic objects. Sensors are provided along the trajectory, which detect the ceramic objects to be tested and control the successive discharges of the electrodes according to a predetermined program. In this way, the objects can be fully tested during the movement and defects in the parts become noticeable in a charge pattern, which is detected by receiving electrodes. Fault signals activate a sorting mechanism, which removes the part found to be defective from the transport mechanism.
An electrode arrangement is known from DE 31 36 538 C2 comprising two electrodes for testing test objects for the presence of holes or cracks, wherein the two electrodes are polarized opposite to each other. One of the two electrodes stands in contact with the test object to be tested and the other electrode is partially adapted to the outer contour of the test object, which is moved past the electrodes by means of a conveyor along a transport path. The voltage applied to the electrodes generates a corona discharge current between the electrodes if the test object has no leak, and a spark discharge current between the electrodes if the test object has a leak, i.e. a microhole or crack.
Furthermore, from DE 10 2013 014 473 A1 there is known a device for testing plastic closure caps for the presence of microholes or microcracks, in which an electrode is designed as a finger of a star wheel, which is inserted into the closure cap as far as the bottom. A counter electrode is placed stationary on the other side of the bottom underneath a transporting mechanism for the closure caps, which has parallel transport belts running in the same direction. The closure caps when moving through the testing mechanism must not become damaged by the star wheel. A typical risk of damage consists in that the inserted finger does not dip into the closure cap, but instead strikes the opening edge. In order to avoid such damage, the closure caps are usually moved under the star wheel jammed seamlessly against one another, so that one finger after another can reach into the abutting closure caps without destruction. The build-up of the closure caps is accomplished in that the closure caps move more slowly in the jamming area in front of the star wheel than the transport mechanism. This can be achieved, for example, by regulating the speed of rotation of the star wheel relative to the speed of advancement of the transport belts.
However, the jamming together of the closure caps needed for their damage-free testing has several drawbacks:                Because of the relative velocity between the transport belts of the transport unit and the closure caps, friction occurs, which can result in damaging of the closure caps, for example, an imprinted image placed on the top side of the closure cap.        The build-up of the closure caps must have a certain minimum length so that under no circumstances can there occur a gap underneath the pin wheel. The minimum length requires a certain structural height of the testing mechanism, which may be a disadvantage when integrating it into the manufacturing facility.        Insofar as closure caps with a guarantee strip or flaps are to be tested, the testing mechanism often has a so-called slitter for making the guarantee strip or a folder for making the flaps in front of it. For cost and space reasons, the testing mechanism and the upstream slitter or folder preferably have a common transport mechanism for the closure caps. The back-up of the closure caps needed for the testing mechanism however must in no case extend as far as the upstream slitter or folder, since these devices might become damaged in this way.        